Oxide coated cathode



Patentedtlan. 12, 1943 OXIDE COATED CATHODE George F. Reyling,Rutherford, N. 5., assignor to Radio Corporation of America, acorporation of Delaware No Drawing. Application November 22, 1941,

Serial No. 420,083

Claims.

My invention relates to cathodes, particularly to cathodes of the typehaving a heated core coated with an alkaline earth compound.

In the manufacture of commercial radio tubes having specified voltageand current ratings for the electrodes, there is the ever present anddifficult problem of making the tubes with uniform ratings. Cathodeemission, which must have a high and uniform level, is sensitive toslight changes in materials and manufacturing procedure and often, forno apparent reason, will suddenly drop below rated values.

An object of my invention is a cathode which has a high uniform level ofelectron emission which is stable and comparatively insensitive tomanufacturing processes.

According to my invention a cathode core is polished to a high gloss sothat its surface has a mirror-like finish. The polished core is coatedin the usual way with alkaline earth compounds, is mounted in the usualway in its radio tube, and is activated. cathodes prepared in thefactory according to my invention consistently have a uniform high levelof electron emission and are not subject to sudden slumps in emission,which are characteristic of poisoned cathodes prepared in the usual waywithout polishing.

Electromechanical means is preferred for polishing the cathode cores.One electrolyte which will remove the outer layers of nickel or nickelalloy cores and leave a brightly polished surface is a mixed water-freesolution of phosphoric and sulfuric acids. With the nickel cathode coreconnected to the positive terminal of a direct current potential in thissolution, the surface of the nickel is removed by electrolytic actionand its surface. in a few minutes, is mirror-like in appearance asdistinguished from the etched or mat surface produced by ordinary acidetching.

Cathode sleeves for radio tubes having small envelopes, of the typecommercially known as the acorn tube, such as No. 954, and shown in theUnited States patent to Salzberg 2.030,18'7, February 11, 1936, havebeen made with considerable success according to my invention. Thecathode sleeve of the acorn tube is .025" in diameter and has a wallthickness of .002" and is made of commercial Grade A nickel having thefollowing composition- Per cent Nickel (and cobalt) 99.15 Iron .20Manganese .20 Copper .20 Silicon .05 All other metals .10

The nickel may, of course, contain cobalt in the small amounts usuallyobtained in refining nickel, less than one percent, or greater amountsof cobalt, up to several percent, usually used for stillening andhardening the nickel. The electrolyte with which particularly goodresults have been obtained contains from 50 to phosphoric acid, 1

the remainder being sulfuric acid. After attaching a nickel tab to theend of the sleeve it is degreased by boiling for five minutes in each oftwo baths of commercial cleaner known as Blakosolv." Seventy-five ofthese sleeves held in a metal clip and immersed in thephosphoric-sulfuric electrolyte are connected to the positive terminalof a voltage source, and a current of 15 amperes at about 15 volts ispassed between the sleeves and a negative electrode in the electrolytefor about 2 minutes. The current density for this particular polishingbath and time of polishing is about 5 amperes per square inch, butsince, in electrochemical reactions, the amount of deposition or removalof metal at an electrode is a function both of time and current density,these two factors may be varied at will. The sleeves are then rinsed inwater containing a wetting agent, such as commercial- Nacconol. Byboiling the sleeves in water containing 01% of this wetting agent, allof the acid is removed from within the sleeves and from cracks,scratches or crevices on the sleeves. An ammonia rinse is desirable iftraces of copper are found deposited on the surface of the sleeve. Theyare then preferably boiled for two five-minute periods in distilledwater, after which they may be rinsed in methanol and dried. Firing incommercial or line hydrogen for ten minutes at 600 C. is believednecessary to displace undesirable occluded gas in the metal and preparethe cathode sleeve for its coating. The cathode is finally coated to adepth of about .003 inch with a spray mixture of barium-stron-- tiumcarbonates. B masking the ends of the sleeves the sprayed coating may beaccurately confined to the center section of the sleeves. After dryingthe coating in air at C. for a few 'minutes the cathode may be mountedwith grids,

anodes and other electrodes in its envelope and, after the usualexhausting and cathode activating schedule, the envelope may be sealedoff.

Cathodes prepared according to my invention have been found, in the 954acorn type tubes, to have an average cathode emission of over 40milliamperes at rated voltages, as compared to less than 35 milliamperesfor a cathode having a core without a polished surface in the same tubewhen operated at the same voltages. The importance of my polishedcathode becomes more significant when the ratio of good to defectivetubes made in the factory is considered. The percent of tubes discardedbecause of emission below rated values with untreated cathodes has oftenrun higher than 50% whereas the percent discarded tubes with cathodesprepared according to my invention is consistently less than 5%.

Cathode sleeves electropolished in a phosphoric-sulfuric acid bathappear under a microscope, having a magnification of about 14 diameters,to be entirely free of surface scratches, cracks, crevices and sharpfissures. While the diffused reflection is negligible and the specularreflection approaches 100%, the gloss cannot be measured in absolutevalues but may be merely compared to an arbitrary standard selected forone material, as explained in the Bureau of Standards research paper RP958, Methods of Determining Gloss," January 1937. The surface I preferand with which I have had good results has a reflectance approachingthat of a conventional silvered plate glass mirror and is easilydistinguished from the sleeve before treatment which, when viewed undera 14 power microscope, has sharp angular crevices, scratches andgrooves. The roughened surface on nickel is produced even by drawingdies that appear to be smooth. Accordingly the term mirror finish" usedthroughout this specification and the appended claims is understood tomean a finish corresponding to the finish that is produced on thesurface of nickel when electropolished in a 70% phosphoric-30% sulfuricacid solution for 2 minutes at 15 volts. There may of course be othersolutions that will produce the same degree of high polish as well asmechanical methods, such as rouge or fine abrasives for producing thefinish.

It is not apparent why nickel cores polished according to my inventionshould be more stable in factory production, nor why the electronemission should be uniform and high. Removal of metal from the surfaceof the core in the polishing bath probably leaves a surface that is freeof oxides and all contaminations and leaves no recesses on the coresurface which may catch and retain contaminating elements. Further theremay be a slight difference in the interaction of the acid upon thenickel and upon the reducing agents, such as silicon and manganese, inthe nickel. The molecules or particles of the reducing agents may thusbe more effectively exposed at the surface of the core and brought intocloser contact with the deposited coating of alkaline earth oxides. Thisaction is evidenced by the fact that the interface between' the core andthe coated oxides is of uniform light gray color, is firmly bonded tothe core, and does not peel from the core as often as in the case ofuntreated drawn nickel wire or tubing. Another characteristic of myimproved cathode is its lower heat radiating property. Since heatasoaaao will not readily radiate from a polished surface, the core mayoperate at a higher temperature for a given power input to the core.Further, the wall thickness of the core is estimated to be reducedbetween 10 and 20% when electropolished as described above. Thinnerwalls improve heat conductivity through the walls and proportionatelyreduce heat loss to the end supports.

Cathodes prepared according to my invention have a high uniform level ofemission and are comparatively insensitive to ordinary contaminationsencountered in handling in radio tube manufacture.

I claim:

1. The method of manufacturing an oxide coated cathode comprisingdrawing a nickel core, polishing said core to a bright mirror finishthat is distinguished from the mat finish of an acid etched nickelsurface or a die drawn nickel surface, washing said core and thencoating the core with an alkaline earth compound.

2. The method of making an oxide coated cathode comprising drawing anickel core to the desired size, polishing said core so that the surfaceof the core has a mirror-like appearance as distinguished from theroughened die marked surface of the core before polishing.

3. The method of making oxide coated cathodes for electron dischargedevices comprising immersing a nickel body in an electrolyte containingabout '10 to percent phosphoric acid and about 30 to 10 percent sulfuricacid, and passing current to said body through the electrolyte, saidcurrent having a density of about 5 amperes per square inch of saidbody, continuing said current for about two minutes, then rinsing saidbody and coating said body with an alkaline earth compound, and finallydecomposing and activating the coating in vacuum.

4. A cathode comprising a nickel core characterized by a smoothmirror-like surface, when optically magnified 14 diameters, asdistinguished from a surface with sharply defined scratches andcrevices, and a coating of alkaline earth compounds on said surface.

5. A cathode comprising a nickel core coated with an adherent layer ofalkaline earth compounds, the surface of the core having a smoothpolished appearance, when optically magnified about 14 diameters, asdistinguished from the appearance of the surface of seamless nickeltubing drawn in the usual way.

GEORGE F. REYLING.

